In certain industrial processes, such as the precipitation section of the Bayer process, it is desirable to monitor the particle size distribution of solids in a slurry or suspension under dynamic conditions and to determine the deviation of the actual particle size distribution from that desired. The invention will be herein described with particular reference to the Bayer process but is not limited to that use.
In the precipitation section of the Bayer process alumina is extracted and purified from bauxite. Bauxite is crushed and ground, then digested at elevated temperature (104.degree.-230.degree. C.) and pressure in a strong solution of caustic soda (80-110 g Na.sub.2 O/liter). The residue, known as red mud, is separated from the solution by countercurrent decantation and filtration. After cooling, the solution is supersaturated with respect to alumina. In the precipitation (crystallization) process the supersaturated solution is seeded with recycled alumina trihydrate fines and agitated in large tanks. The productivity of the Bayer process is limited by the slowest step, the precipitation of the alumina trihydrate from the liquor. After precipitation, the alumina is calcined at temperatures up to 1300.degree. C. and sold typically for commercial smelting to aluminium.
The goal of the precipitation section is to maximise alumina yield whilst maintaining product quality (particle size and strength) and seed balance. Control variables in precipitation are temperature, starting alumina/caustic ratio, seed charge, caustic concentration, holding time and impurity levels (particularly organics).
It is desirable to minimise the proportion of fines (minus about 45 microns as well as minus about 20 microns) in product alumina. These fines cause dust problems during handling operations and they cause significant flow and segregation problems in aluminium smelters. Fines are generated in the precipitation, calcining and handling stages of the plant. Improved control of the precipitation stage should produce less fines in precipitator product and a stronger alumina (Sang, J. V., "Factors affecting the attrition strength of alumina products", Light Metals, 1987, 121-127) which will in turn generate less fines in downstream calcining and handling.
The optimisation of alumina precipitation would benefit from the development of suitable on-line particle size analysers to monitor the proportions of both coarse and fine alumina at a number of cut-off points, for example at 20,45,75 and 100 microns. Measurements preferably need to be made under the following plant conditions: caustic concentration 150 to 250 g NaOH/liter; temperature 60.degree. to 80.degree. C.; solids content up to 35 wt %; scale build-up rates of about 10 mm /week; and variable air bubble concentration and size distribution.
At present there are no commercially available on-line particle size monitors capable of this measurement.
The conventional method of measuring particle size distribution is to remove samples from the streams of interest and to perform screen analyses on these samples. A screen analysis involves a series of procedures by which a measurement is made of the proportion of the sample that remains on each of several screens having progressively smaller openings of known size. While this kind of measurement can provide a reasonably accurate determination of particle size distribution above about 45 microns, it is representative only of the particular sample taken, and cannot accurately and reliably indicate either the average condition in the flowstream over a period of time, or the changes that occur between sampling. Also the method is not applicable to fine fractions below 45 microns, and is labour intensive and time consuming and therefore does not lend itself to either manual or automatic control.
There are three commercially available on-line particle size analysers which are finding significant use in the control of grinding circuits in the metalliferous mineral industry. However, these analysers are not suitable for use in alumina precipitation circuits, primarily because of air bubbles, scaling, caustic and high temperatures. These three on-line particle size analysers are based on ultrasonic attenuation (Autometrics PSM-400), a scanning laser microscope (Lasentec Par-Tec 200/300) and a reciprocating caliper (Outokumpu PSI-200). Alternative systems based on laser diffractions operate only on highly dilute solutions (&lt;5 g solids/liter) and are not suitable for on-line use. The on-stream laser scanning microscope (the Lasentec Par-Tec 200/300) does not require dilution of the process stream (J. Hokanson, "In-line particle size measurement for improved process control in the mineral processing industry", Light Metals 1991, 39-42). In this analyser, the sample is scanned at constant speed by a high intensity laser beam and the reflected light is measured. Particles outside the focus of the laser beam are discriminated against on the basis of the rise time of the reflected light pulse. The main disadvantages of this technique are:
(i) it measures a very small sample and so flow conditions at the window affect the results; PA1 (ii) it does not operate in the presence of scale; PA1 (iii) it requires a transparent fluid; and PA1 (iv) the sensitivity to surface characteristics (shape and reflectivity) and air bubbles, while also being inaccurate on fine particles. PA1 2.alpha.=absorption coefficient PA1 A.sub.0 =initial intensity of wave PA1 x=distance travelled by wave PA1 .lambda.=wavelength of ultrasonic wave, PA1 c=volume concentration of solids, PA1 k=angular wavenumber of ultrasonic wave in water, PA1 .gamma.=ratio of densities of particles and water, PA1 .omega.=angular frequency of ultrasonic wave, ##EQU2## v=kinematic viscosity of water, ##EQU3## PA1 (i) measuring the velocity of ultrasound on transmission through a sample of the solution; PA1 (ii) measuring the attenuation of ultrasound on transmission through the sample; PA1 (iii) measuring the attenuation of electromagnetic radiation on transmission through the sample to obtain a measure of the density of the sample; PA1 (iv) deriving from the measure of density and the measure of said velocity an estimate of the concentration of solids in suspension; PA1 (v) deriving from the measure of density and the measure of said velocity an estimate of the solute concentration; and PA1 (vi) deriving from the measures of ultrasonic attenuation, ultrasonic velocity and density a measure of particle size distribution. PA1 first means for providing a first signal indicative of the velocity of an ultrasound beam of predetermined frequency directed through a sample of the solution; PA1 second means for providing a second signal indicative of the attenuation of ultrasound on transmission through the sample; PA1 third means for measuring the attenuation of electromagnetic radiation on transmission through the sample and providing a third signal indicative of the density of the sample; PA1 fourth means for deriving fourth and fifth respective signals indicative of the concentration of solids in suspension and the solute concentration from said first and third signals; and PA1 fifth means for deriving particle size distribution from said first, second and third signals. PA1 (i) measuring the velocity of ultrasound on transmission through a sample of the solution; PA1 (ii) measuring the attenuation of electromagnetic radiation on transmission through the sample to obtain a measure of the density of the sample; and PA1 (iii) deriving from the measure of the density and the velocity an estimate of the concentration of solids in suspension. PA1 (i) means for measuring the velocity of ultrasound on transmission through a sample of the solution; PA1 (ii) means for both measuring the attenuation of electromagnetic radiation on transmission through the sample and for producing a signal indicative of the density of the sample; and PA1 (iii) means for deriving signals indicative of the concentration of solids in suspension from the measurements of said density and said velocity. PA1 (i) measuring the velocity of ultrasound on transmission through a sample of the solution; PA1 (ii) measuring the attenuation of electromagnetic radiation on transmission through the sample to obtain a measure of the density of the sample; and PA1 (iii) deriving from the measure of density and the measure of said velocity an estimate of the solute concentration. PA1 (i) means for measuring the velocity of ultrasound on transmission through a sample of the solution; PA1 (ii) means for measuring the attenuation of electromagnetic radiation on transmission through the sample and producing a signal indicative of the density of the sample; and PA1 (iii) means for deriving signals indicative of the solute concentration from the signals indicative of said density and said velocity. PA1 a.sub.0, a.sub.1, a.sub.2, a.sub.3, b.sub.0, b.sub.1 are constants, PA1 .rho. is the density, PA1 I and Io are the gamma-ray intensities with and without the suspension respectively, and PA1 T is temperature. PA1 (a) ultrasonic transmission attenuation at a number of ultrasonic frequencies generally within the range 20 kHz to 20 MHz (A.sub.1, A.sub.2, . . . ); PA1 (b) total ultrasonic transmission transit time in the probes and suspension at each frequency (T.sub.1, T.sub.2, . . . ); PA1 (c) ultrasonic probe reflection amplitude at each frequency (P.sub.1, P.sub.2, . . . ); PA1 (d) ultrasonic probe reflection transit time at each frequency (R.sub.1, R.sub.2, . . . ); PA1 (e) gamma-ray attenuation in the suspension (I/Io) where I and Io are the gamma-ray intensities with and without the suspension respectively; and PA1 (f) temperature of the suspension (T).
The reciprocating caliper device (Outokumpu PSI-200) uses a simple and direct measurement technique to determine the largest particle in the caliper. The device will not operate in the presence of scale and it cannot measure the fine particle size fractions.
The ultrasonic device (Autometrics PSM-400) determines the fraction of particles of size above a particular cut point (usually about 75 microns) by measuring ultrasonic attenuation at two frequencies. One frequency is chosen where attenuation is not greatly affected by particle size and the second frequency is chosen to be sensitive to particle size. Combining these results allows one to derive a single point on the size distribution curve, provided that the shape of the size distribution curve remains fairly constant. Measurements can be made on water-based slurries containing up to 30 wt % solids although air bubbles need to be eliminated before measurement.
The Autometrics PSM system, however, is not suitable for use in alumina precipitation plants as it is unable to correct for changes in caustic concentration, it is unable to determine the concentration of fine (minus 20 micron) particles and there is no correction for the effect of scaling or temperature.
The theory for ultrasonic theory attenuation methods shows that viscous and scattering losses provide the two main regions where direct calculation of size from the observed attenuation is possible. If a pulsed beam of ultrasonic waves is transmitted through a sample slurry stream, the intensity of the transmitted beam is given approximately by: EQU A=A.sub.0 exp(-2.alpha.x) (1)
where
The absorption coefficient is determined by two primary mechanisms, viscous and scattering losses. Viscous losses are associated with the relative movement of liquid and solid. The particles vibrate in response to the ultrasonic wave but with a phase lag and different amplitude. Extremely small particles tend to move in phase with the fluid and losses are very small. As size increases the particles tend to lag more and more behind the movement of the fluid and the loss per particle increases, but at the same time the solid-liquid interface area and therefore the loss per unit mass decrease. These opposing factors result in a viscous absorption maximum below about 3 microns particle size for frequencies above 800 kHz, as shown in FIG. 1 in which the calculated dependence of the absorption coefficient on particle size at different frequencies is plotted for suspensions in water (Riebel, U. and Loffler, F., U.S. Pat. No. 4,706,509 Nov. 17, 1987). The ratio of particle density to fluid density and the fluid velocity are also important in determining viscous loss.
The second loss mechanism is the scattering of energy due to the absorption of a small amount of energy from the directed beam by each particle, and its subsequent radiation away from the point of absorption.
For ultrasonic wavelengths much greater than particle radius, it is possible to express the relationship between .alpha. and the properties of the solids and liquid by the equation: ##EQU1## where r=particle radius
This equation is valid up to about 20% solids, above which the dependence on c becomes more complex.
The first term in equation (2) represents the attenuation due to scattering loss and the second due to viscous loss. For a given frequency, at very small particle sizes the viscous loss is predominant but as the size increases it becomes insignificant and the scattering loss becomes important.
For the case where .lambda.&lt;&lt;2.pi.r (generally for particles well above 100 .mu.m diameter and for frequencies of 1-10 MHz), diffraction losses become important and equation (2) must be replaced by the expression ##EQU4##
Relevant patents covering ultrasonic attenuation methods of particle size determination are U.S. Pat No. 3,799,070 (Cushman et al), U.S. Pat. No. 4,706,509 (Riebel and Loffler) and U.S. Pat. No. 5,121,629 (Alba). U.S. Pat. No. 3,799,070 describes the method used in the Autometrics PSM-400 analysers in which only two measurements are made, namely ultrasonic attenuation at two frequencies. The signals resulting are processed electronically to provide information on particle size and percent solids by volume in the slurry. U.S. Pat. No. 4,706,509 (Riebel and Loffler) effectively extends the Cushman et al patent by measuring the ultrasonic attenuation at a plurality of frequencies and so is able to derive a wider range of particle size data. U.S. Pat. No. 5,121,629 (Alba) also involves measuring ultrasonic attenuation over a selected frequency range. Alba describes various methods of deriving particle size distributions by comparing the measured and calculated attenuation spectra.
The velocity of ultrasound in suspensions has been shown by Harker and Temple (Journal of Physics D: Applied Physics 21 (1988) 1576-1588) to be a complex function of volume fraction, particle radius, fluid viscosity, frequency and compressibility. The effect of particles on the velocity of ultrasound in a suspension is best explained by an example. FIG. 2 shows the calculated velocity as a function of particle size for 10 wt % silica in water for various ultrasonic frequencies. For the larger particle sizes the wave propogation velocity is the same as that in the pure suspending fluid, although ultrasonic attenuation may be high. For the very small particle sizes, the particles tend to move in phase with the fluid in response to an ultrasonic wave. The suspension as a whole can be regarded as a continuum with respect to ultrasonic propagation. As a result, ultrasonic attenuation is very low, but the wave propagation velocity in the suspension may differ considerably from the wave propagation velocity in the pure suspending fluid. For intermediate fine particle sizes, between about 0.2 and 30 microns (for frequencies 50 kHz to 10 MHz), there are substantial velocity differences as a function of frequency. However, these velocity differences cannot be used to determine particle size without separate measurement of solids weight fraction and fluid characteristics.